Genetic Structure of Ralstonia solanacearum and Ralstonia pseudosolanacearum in Brazil

Abstract

Bacterial wilt-causing Ralstonia threaten numerous crops throughout the world. We studied the population structure of 196 isolates of Ralstonia solanacearum and 39 isolates of Ralstonia pseudosolanacearum, which were collected from potato- and tomato-growing areas in 19 states of Brazil. Regardless of the species, three groups of isolates were identified. One group encompassed R. pseudosolanacearum isolates. The other two groups comprise isolates of R. solanacearum (phylotype II) split according to geographic regions, one made of isolates from the North and Northeast and the other made of isolates from the Central, Southeast, and South regions (CSS). Among the isolates collected in CSS, those from tomato were genetically distinct from the potato isolates. The genetic variability in the population of R. pseudosolanacearum was lower than that of R. solanacearum, suggesting that the former was introduced in Brazil. Conversely, the high genetic variability of R. solanacearum in all regions, hosts, and times supports the hypothesis that this species is autochthonous in South America, more precisely in Brazil and Peru. For R. solanacearum, higher variability and lower migration rates were observed when tomato isolates were analyzed, indicating that the variability is caused mainly by the differences of the local, native soil population. The North subpopulation was distinct from all others, possibly because of differences in environmental features of this region. The proximity of some geographic regions and the movement of potato tubers could have facilitated migration and therefore low genetic differentiation between geographic regions. Finally, geography, which also influences host distribution, affects the structure of the population of R. solanacearum in Brazil. Despite quarantine procedures in Brazil, increasing levels of trade are a threat to biosecurity, and these results emphasize the need for improving our regional efforts to prevent the dispersal of pathogens.

Contrasting genetic diversity and structure among Malagasy Ralstonia pseudosolanacearum phylotype I populations inferred from an optimized Multilocus Variable Number of Tandem Repeat Analysis scheme

“…A striking result is the singularity of the most prevalent Malagasy isolates belonging to sequevar 18 (corresponding to cluster 1). Indeed, despite the fact that sequevar 18 is distributed worldwide [37,[67][68][69][70][71][72][73]75], our study revealed no genetic links between Malagasy isolates and worldwide isolates (South-West Indian Ocean, Africa, Americas, Asia isolates). The closest haplotype (one strain) was a double-locus variant from Guadeloupe.…”

“…Interestingly, sequevar 31 isolated in the lowlands and up to altitudes above 1000m in Réunion [37], as well as in Côte d'Ivoire [67], exhibited a similar environmental distribution. So far, sequevar 18 has always been reported from lowlands in the Americas [68][69][70], Africa [67], Asia [71][72][73], Oceania [74] and on islands in the South-West Indian Ocean [37,75]. In contrast, even though a very limited number of isolates was used in our study, sequevar 33 was only isolated at moderate and high elevations (above 900m) and sequevar 46 was only isolated in lowlands (below 300m).…”

On the assessment of the sources of inoculum of bacterial wilt in Brazil

“…The occurrence of R. solanacearum in soil, water and the rhizosphere of cultivated and non-cultivated plants in all regions in Brazil provides additional support to the hypothesis that the country is a putative center of origin of this bacterial species (Wicker et al 2012;Santiago et al 2020). In addition to the practical implications of the findings of the present study, i.e.…”

Molecular characterization and aggressiveness of the Ralstonia solanacearum species complex from Eucalyptus spp. in Brazil

“…Planting of pathogen‐free rooted cuttings and use of resistant genotypes are the best management strategies to control BW in Eucalyptus (Fonseca et al., 2016). However, a high genetic variability of the RSSC found in Brazil (Fonseca et al., 2014; Santiago et al., 2020) may hamper the effective development of resistant clones in breeding programmes. The correct taxonomic classification of the pathogen and a good knowledge of its genetic and aggressiveness variability is a key factor when undertaking epidemiological studies of this species complex that could contribute to disease management.…”

“…In addition, they allow the selection of the most aggressive isolates to be used in inoculations for screening genotypes with effective resistance. Because of the high genetic diversity of R. solanacearum associated with solanaceous plants (Santiago et al., 2020) and the recent detection of R. pseudosolanacearum also causing BW in eucalypt (Freitas et al., 2020), we hypothesized that a study of the genetic variability of the RSSC population associated with eucalypt in Brazil could provide important information to help guide studies aimed at establishing appropriate disease management strategies. As such, this work aimed to molecularly characterize a large collection of RSSC isolates obtained from Eucalyptus , to determine to which phylotypes and sequevars they belong, to gain insights into their geographic distribution as well as to assess the aggressiveness variability of a select group.…”